This invention is in the field of surgical implants, and relates to devices and methods for repairing hyaline or meniscal cartilage in joints such as knees, hips, fingers, shoulders, etc.
In joints that are lubricated by synovial fluid, hyaline and meniscal cartilage segments provide smooth, slippery, lubricated (or lubricious) surfaces that enable bones to move and slide, relative to other bones. “Hyaline” cartilage refers to the types of cartilage segments that are affixed, in relatively thin layers, directly to bone surfaces (often called condyles). Background information on hyaline cartilage, and on surgical implants for replacing injured or diseased hyaline cartilage, is available from various sources, including several prior patent applications by the same inventor herein, such as Ser. Nos. 11/390,539 (“Implants for replacing hyaline cartilage, with hydrogel reinforced by three-dimensional fiber arrays”), 11/105,677 (“Hydrogel implants for replacing hyaline cartilage, with charged surfaces and improved anchoring”), and 10/071,930 (“Cartilage repair implant with soft bearing surface and flexible anchoring device”).
Meniscal cartilage is more complex. Each knee joint has two meniscal segments, which are arc-shaped segments with triangular cross-sections, depicted in any textbook on anatomy. These meniscal segments are positioned on the left and right sides of each knee (referred to by physicians as the “medial” (inside) and “lateral” (outside) positions, and they help stabilize the femoral runners against “sideways” motion. Each meniscal wedge has two smooth and lubricious surfaces (a smooth lower surface, which is roughly horizontal, and a smooth upper surface, which is slanted and which curves sharply around the interior curved upper surface of the rounded wedge-type segment). These meniscal wedges are made of a specialized type of “fibrocartilage”; rather than being affixed, like a thin coating layer, on a bone surface they have anchoring ligaments, both at their tips (which attach to bone protrusions near the center of a tibial plateau), and around their peripheral surfaces (to the tendons and ligaments that form a “capsule” which encloses the knee and holds in the synovial fluid which lubricates the joint). Additional information on meniscal cartilage (as used herein, that term includes a structurally similar set of “labral” cartilage segments in hip and shoulder joints), and on the design of implants for replacing or repairing damaged meniscal or labral cartilage segments, is available from sources such as U.S. patent application Ser. No. 11/471,090, “Multi-part implants for combined repair of hyaline and meniscal cartilage in joints”.
Joints that contain hyaline and/or meniscal cartilage (which includes labral cartilage) are referred to herein as “synovial” joints, since they are lubricated by synovial fluid. These joints can alternately be called “articulating” joints, because they involve joints having bone surfaces which move, relative to each other, in a manner referred to as “articulating” motion. The types of cartilage (and joints) of interest herein specifically exclude: (1) cartilage in spinal discs, which do not have any sliding surfaces, and which have a very different structure and bone-anchoring system, which actively prevents any sliding or shearing motions, since any such motion could severely injure the spinal cord; and, (2) other non-sliding, “non-articulating” cartilage, which is present in various body parts such as the nose, ears, windpipe, etc. Neither of those two types of cartilage (in spinal discs, or in ears, noses, windpipes, etc., not need to withstand the types of loadings and stresses that are imposed on synovial (i.e., articulating) joints. Therefore, implants which are designed to replace cartilage in spinal discs, or in ears, noses, windpipes, etc., do not require the types of specialized anchoring systems disclosed herein.
All implants of interest herein are specifically designed to be “substantially flexible”, to a point which will enable “minimally invasive” surgical implantation, since flexibility can enable an implant device to be at least partially curled or rolled up, compressed, or otherwise flexed into a shape that can pass through a smaller incision than would be required by a non-flexible implant. Anything that can minimize the amount of cutting and disruption of soft tissues and vasculature, in and around a joint that is being repaired, will minimize damage to the surrounding tissues, thereby benefiting the patient and reducing pain, recovery times, risks of infection, etc.
The optimal type of minimally-invasive surgery on joints is arthroscopic surgery, in which all necessary instruments and devices enter a joint via small slits. In this type of surgery, any implant devices should be designed to allow passage through an arthroscopic insertion tube having the smallest practical diameter. Accordingly, the implants described herein preferably should be not merely slightly flexible; instead, an optimal implant should be flexible enough to be rolled up into a cylindrical configuration, to allow an implant to be inserted into a joint via an arthroscopic insertion tube.
If not adequately defined, the term “flexible” is inherently indefinite; for example, it can be argued that virtually anything that is not brittle or friable is (or can become) flexible, if enough force is applied. Therefore, a set of practical limits and “benchmark” standards is established and used herein, to define “flexible” (as that term is used in the claims), and to determine whether some particular implant has sufficient flexibility, on a practical level, to render it suited for use as disclosed herein.
Accordingly, a candidate implant device is deemed to be “flexible”, as used in the claims, if the device (as manufactured and assembled in a form that will be removed from a sealed sterile envelope by a surgeon, immediately before implantation during a surgical procedure), meets either or both of the two following criteria:
(1) if it can be flexed (or curled, rolled, bent, etc.), without requiring tools, into a configuration that has an “angle of displacement” of at least about 70 degrees. By way of illustration, if one edge of the implant is held horizontal, on the surface of a table, the opposing edge must be capable of being lifted to an angle of at least 70 degrees from horizontal, which is equal to 20 degrees short of completely vertical.
(1) if it can be flexed, without requiring tools, into a configuration where its “width” (i.e., its smallest dimension, when looked at from a “top view” or “plan view”) is reduced to about 80% or less of its width in a non-flexed, relaxed state. By way of illustration, if a femoral runner or a meniscal wedge can be temporarily “straightened out”, from a curved and relatively semi-circular shape into a more linear shape that can be pushed into a joint via an insertion slit or tube, it can enable the insertion of the femoral or meniscal implant into a joint, with less damage to surrounding tissues.
If an implant as described herein is designed for replacing hyaline cartilage (which is relatively thin), it preferably should surpass those minimum levels of flexibility, and the implant should be capable of being rolled into a cylindrical configuration, for implantation via an arthroscopic insertion tube.
Shape-memory and Super-elastic Materials, and Nitinol
Since high levels of flexibility will be required for arthroscopic use of the implants disclosed herein, three specific terms of art in the field of materials science should be introduced and briefly explained. These three terms are shape-memory materials, super-elastic materials, and nitinol.
In general, “shape-memory materials” (SMM's, which includes various polymers as well as certain types of alloys) include any materials that fall within either of two somewhat different functional definitions.
Under the first definition, if a material can be deformed (such as by bending, stretching, etc.) in some way that appears to be stable, under some particular set of conditions, but if the material will return to its manufactured shape without suffering any permanent damage, when subjected to different conditions, then the material is classified as a “shape-memory material”. A common parameter that is used to manipulate shape-memory materials, in ways that make convenient and valuable use of their “shape-memory” trait, is temperature.
For lack of a better descriptive term, the phrase “shape-memory materials” also acquired a second functional definition. If a certain alloy or polymer undergoes some type of “phase transition” which leads to a notably different type of physical performance or behavior, when subjected to a certain type of operating condition or parameter, and then it returns to its “normal” performance or behavior when returned to “normal” conditions, the term “shape-memory material” is often used as a label for that type of material, regardless of whether the different performance actually involves shape. This convention apparently arose when it was discovered, during the 1930's, that wires made of certain types of copper-zinc alloys would shrink, in length (which is indeed a change in shape), when heated; these types of wires came to be used in robotics and toys, as “muscle wires” that would contract, in length, when a current was applied to such wires in a way that caused heating of such wires.
A subsequent development that became of major medical importance arose when it was discovered, in the 1960's, that certain types of alloys containing nickel and titanium had an unusual behavior. Those alloys were called “nitinol” alloys (pronounced NIGHT-in-all), as a spliced acronym that combines the first letters from nickel, titanium, and “Naval Ordnance Laboratories”, the federal research center where nitinol alloys were discovered. Nitinol alloys undergo a temperature-dependent transition that is the opposite of what occurs in most types of alloys and polymers. Most non-rigid alloys and polymers tend to become softer, and more flexible and pliable, when they are heated to higher temperatures. Nitinol alloys become of interest in medical devices, because they can do the exact opposite. At normal human body temperatures, nitinol alloys are in an “Austenite” crystalline form, which is relatively stiff. However, if a nitinol device is chilled in cold water (such as saline slush), it makes an entirely reversible transition to a “Martensite” crystalline form, which is substantially more flexible and pliable.
As a result of that unusual behavior, various types of medical devices are made of nitinol, such as stents (devices for holding blood vessels open, in people who suffer from partially blocked or occluded arteries such as in the heart or neck). These can be implanted and used as follows. If a stent, made of nitinol in the form of a cylindrical wire mesh, is chilled to a “Martensite” temperature (such as by immersing it in cold water), the stent can be compressed into a relatively small diameter that will fit inside a catheter tube, which can be “snaked” into a patient's body via a small incision, such as into a femoral artery. The stent can be kept chilled, while it remains in the catheter tube, by using cold water circulating through special channels in the catheter. After the stent reaches a blood vessel that needs to be unclogged, the catheter tube is withdrawn, allowing blood and surrounding tissues to warm the stent back up to its stiffer “Austenite” state. As that warming process occurs, the stent will expand back into its larger, unstressed, manufactured diameter, which will correspond to the inside diameter of the artery segment that needs to be kept open.
These types of nitinol devices, and the transitions they undergo at differing temperatures, are described and shown in more detail in numerous sources, including a website (www.nitinol.info) run by a company called Nitinol Devices and Components (NDC). Several short videos (about 1 minute each), which visually depict how nitinol alloys and devices behave, are available at www.nitinol.info\pages\technology.html. In addition, a review article by D. Stoeckel, “Nitinol Medical Devices and Implants”, presented at the SMST 2000 Conference, is available at www.nitinol.info/pdf_files/stoeckel—1.pdf.
Accordingly, nitinol devices will not make self-directed transitions into shorter or longer lengths, or other different shapes, when chilled or heated. However, since they become more pliable and “workable” when chilled, they can be readily manipulated into useful shapes (for an implantation process or other purpose) at cold temperatures, and they will then return to a stiffer and stronger manufactured state and geometry, when allowed to warm up to body temperature. As a result, they are usually included within the class of materials called “shape-memory materials”.
The term “super-elastic material” is broader, and it does not have a precise definition. As implied by the term “super”, it includes materials with one or more elastic behaviors that would be regarded as super or superb (which implies especially useful, valuable, and somehow different and better), when compared to conventional elastic materials. In the field of metals, conventional elasticity can be represented and exemplified by long, thin, flexible pieces of stainless steel, or by the types of steel alloys used to make metal springs. In plastics and polymers, conventional elasticity is represented by latex rubber, silicon rubber, rubber bands, etc. Accordingly, “super-elastic materials” include materials that can substantially outperform those types of conventional materials, in one or more ways that involve elasticity. Since “shape-memory materials” that respond to temperature changes, and “muscle wires” that become either shorter or longer when electric currents are passed through them, both fall within that definition, those are often referred to as types of super-elastic materials.
One other point should be noted. In nearly all cases of interest herein, a device made from a shape-memory material usually will seek to return to a certain shape (which will be determined by the manufacturing process), when it returns to a “final” temperature (which will be body temperature, for any surgical implant) or other operating condition. This distinguishes shape-memory devices from items such as rubber bands. A rubber band is elastic, and it will return to a certain length, after any tension that caused it to take an elongated shape has been removed. However, a typical rubber band that has a substantial length will not attempt to return to a certain specific shape. If dropped onto a flat surface, it can come to rest in a relatively straight or oval-like configuration, or it can curve in either a right or left direction, without any substantial stresses arising within the rubber that makes the rubber band.
By contrast, in all cases of interest herein, a shape-memory device will have a predetermined shape, which must be created during a manufacturing operation (which can include various annealing, curing, treating, or other shape-imparting or shape-modifying steps). The device will then seek to return to that predetermined shape. This does not imply that the device must and will always return to exactly its manufactured shape; nevertheless, it will seek to do so, and any shape alterations that may be imposed on the device, by external mechanisms or forces (such as anchoring pins, an adhesive that is used to bond the material to another surface, etc.), will create some level of internal stresses within the shape-memory or super-elastic device.
Accordingly, proper design of a surgical implant made of a shape-memory or super-elastic material must take into account the final shape that the device will take, after it has been implanted in a particular location. Some implants are intended to impose mechanical forces on body parts or mechanical components that contact an implant; this is comparable to installing a spring-loaded device inside a mechanism. However, if creating that type of force is not the intent of a shape-memory or super-elastic implant device, then the implant should be manufactured with an unstressed shape that is as close as possible to the final shape the implant will take, after it has been implanted.
That is a brief introduction to a complex field of materials science. Much more information on these types of materials is available in books such as Otsuka and Wayman, editors, Shape Memory Materials (Cambridge Univ. Press, 1999), and from an organization called Shape Memory and Superelastic Technologies (SMST), www.smst.org. A surgeon does not need to be an expert in this field of materials science, in order to be able to use and appreciate surgical devices that incorporate and use these types of materials. If a surgeon has a working knowledge of what these materials and devices can accomplish, and how they will perform when used in surgical implants, that is sufficient.
Returning to the subject of nitinol alloys, it was initially believed, by the Applicant herein, that certain types of rims or other anchoring components made of nitinol alloys would be ideal, for cartilage-replacing implants, because the use of nitinol alloys would allow them to become much more soft and flexible, by using a chilling process, during insertion into a joint that is being surgically repaired. However, additional research by the Applicant has identified an important obstacle to such use of nitinol alloys, in implants that will remain in a patient's body for an extended period of time. That obstacle involves a risk of corrosion, which is believed to arise primarily in areas where nickel atoms cluster together in “nickel-enriched” clusters or “pockets” that can have molecular structures and/or “lattice ratios” such as Ni3Ti. The bonds between adjacent nickel atoms are not as strong as the bonds between nickel and titanium atoms. As a result, during the manufacture of a nitinol component, if small pockets of material are formed that have nickel content greater than 50%, the nickel atoms in those pockets can be leached out, over a span of months or years, in ways that can lead to corrosion, cavities, and structural weakness.
It has been discovered, through testing, that a nitinol manufacturing process known as “Quick Cool with No Reheat” provides more corrosion-resistant nitinol alloys than a different process known as “Cool Down Slowly”. Accordingly, nitinol alloys have been approved for use in some medical devices that are left in place for years, such as certain types of stents that help keep arteries open in patients who suffer from clogged arteries.
However, since the types of arthroscopically-insertable flexible implants being developed by the Applicant herein, for orthopedic use in load-bearing joints such as hips or knees (where any such implants will need to comply with stricter design requirements and constraints, compared to uses in non-load-bearing locations, such as stents) already have a number of novel and even pioneering features, when compared to conventional orthopedic implants that are in use today (as exemplified by conventional “total knee replacement” implants), this new and innovative “technology platform” is not well-suited for introducing new component and material selections that might trigger extensive additional long-term clinical testing requirements. Those types of long-term testing requirements could lead to severe problems and delays, especially if the main goal of such long-term clinical trials would be to ensure that a certain type of component material will not slowly corrode, over a span of a decade or more, in a mammalian joint.
Therefore, the Applicant herein began studying alternate types of candidate reinforcing devices, using materials that have long track records of biocompatibility with biological fluids and tissues, and which do not pose any risks or questions of potential slow and gradual corrosion. The results of those efforts are described below, as part of this invention.
However, it also should be noted that the use of nitinol, in cartilage-replacing implants designed for permanent implantation (in this context, phrases such as “long term” generally refer to time periods greater than at least 5 or 10 years, while “permanent” refers to the remaining life of a patient), might remain as a completely viable approach, if any such nitinol component will be completely embedded within a polymeric material that will effectively “seal in” (or entomb, or similar terms) the nitinol component, in a way that will prevent any nitinol from ever being contacted, in any significant quantities, by body fluids. That is indeed the design of various types of implants described and illustrated herein; accordingly, the use of nitinol anchoring rims, in such devices, remains as a potentially feasible, practical, and approvable design approach, in such implants.
Accordingly, one object of this invention is to disclose improved designs and constructions for flexible surgical implants that are designed and suited for arthroscopic repair and replacement of hyaline and/or meniscal cartilage, in synovial joints.
Another object of this invention is to disclose improved devices, assemblies, and methods for anchoring, to bone surfaces in synovial joints, flexible surgical implants which are designed for arthroscopic repair and replacement of hyaline cartilage.
Another object of this invention is to disclose improved devices, assemblies, and methods for anchoring flexible surgical implants designed for arthroscopic repair and replacement of damaged meniscal or labral cartilage.
These and other objects of the invention will be become more apparent through the following summary, drawings, and detailed description.